Frequency converters often have a small control panel with a limited number of keys and a small display. These are used to input certain user definable parameters and to show, for example, the status of the frequency converter.
The desire to keep the size of the display small and to limit the number of the keys, for example, for reasons of reliability, simplicity and compactness, imposes limits for the user interface available to input symbol strings, such as editing texts. A text editor is, however, useful for naming the controlled devices, for instance. After all, the control panel should be robust and suitable for heavy professional use.
The control panels and text editing functions of power electronic apparatuses are presented in DE 102008005880 A1 (SIEMENS AG), DE 102007039188 A1 (SIEMENS AG), DE 102007039165 A1 (SIEMENS AG), WO 0207298 A1 (YASKAWA DENKI) and EP 0443585 A2 (MITSUBISHI ELECTRIC CORP).
Of the above documents, WO 0207298 A1 discloses a method for enhancing the operability by decreasing the number of operations required for altering the setting of parameters of an inverter. The parameter number of each parameter of an inverter includes a rough function number A1, A2, B1, . . . , and a detailed function number 00, 01, 02, . . . which can be altered independently. When the set value of a parameter of the inverter is altered, a rough function of that parameter is selected at first using the rough function number, and a parameter is retrieved roughly. Subsequently, a detailed function is retrieved and indicated, and the set value of the parameter is altered. A parameter having a set value to be altered can thereby be indicated with a small number of operations.
The above document does not, however, provide a technique of inputting symbol strings, such as text strings.
One possibility for editing and inputting symbol strings is to provide a scrollable symbol list, from which the input symbols can be selected one by one. This solution is very intuitive and easy to use, but it is not practical in case there is a desire to use a great number of potential symbols. This is because then the symbol list is very long and it is tiresome and time-consuming to scroll through the list to select the desired input symbols.